Method of preparing inorganic aerogels

ABSTRACT

SILICA AEROGELS ARE PREAPRED BY HYDROLYZING A LOWER ALKYL ORTHOSILICATE IN AN ALIPHATIC ALCOHOL HAVING 1-4 CARBON ATOMS WITH ONE TO FIVE TIMES THE STOICHIOMETRIC QUANTITY OF WATER REQUIRED TO EFFECT HYDROLYZATION. IN A SEPARATE VESSEL, A SUPPLEMENTARY AMOUNT OF THE SAME ALCOHOL IS ADDED AND THE CONTENTS OF BOTH VESSELS ARE GRADUALLY HEATED TO A TEMPERATURE ABOVE THE CRITICAL POINT. THE PRESSURE IS GRADUALLY RELEASED AND WATER AND ALCOHOL VAPORS ARE WASHED OUT WITH DRY INERT GAS. THE AUTOCLAVE IS THEN RESEALED AND COOLED TO ROOM TEMPERATURE AND THE RESULTING AEROGEL IS COLLECTED.

June 27, 1972 s.J.1r-:|cHNER r-:rAL 3,672,833

METHOD OF PREPARING INORGANIC AEROGELS Filed Feb. 8, 1971 HTH/J.

United States Patent O 3,672,833 METHOD OF PREPARING IN ORGANIC AEROGELS Stanislas Jean Teichner and Gilbert Andre Nicolaon, Lyon, France, assignors to Etat Francais represente par le Ministre des Armees Delegation Ministerielle pour IArmement, Paris, France Continuation-impart of application Ser. No. 779,976, Nov. 29, 1968. This application Feb. 8, 1971, Ser. No. 113,246

Int. Cl. C01b 33/16, 33/14 U.S. Cl. 23-182 R 20 Claims ABSTRACT F 'I'HE DISCLOSURE This application is a continuation-in-part of our copending application Ser. No. 779,976 led Nov. 29, 1968 and now abandoned.

This invention relates to methods of preparing silica aerogels, i.e. silica gels wherein a gas or vapour is included as the continuous phase, that is, as the dispersion medium.

One aim of the invention is to develop methods of the aforementioned kind which are in accordance with practical requirements, inter alia with regard to their speed and the cost price, quality and special properties of the products, which can be used in novel manners.

It is already known to manufacture silica aerogels by precipitating the gel by acidifying a solution of sodium silicate, washing the gel in distilled water until all the acid and salts resulting from neutralisation have been eliminated, by replacing the water as completely as possible by an organic solvent and by drying the solventimpregnated gel in an autoclave under hypercritical conditions.

However, the operation of washing the gel to remove the salts and the operation of replacing the water by the solvent are extremley long and, as a result, are extremely expensive in labour costs.

To clarify ideals, we note that the gel, after being precipitated by acidification from the silicate solution, needs to be washed aboutpfty timesrinorder to eliminate all traces of salts and acid.

It is also known to prepare silica gels by forming a mixture consisting essentially of ethyl orthosilicate, water in a volume ratio of water to ethyl orthosilicate of at least 2, a strong mineral acid, other than such acid that is reactive with said silica, in said water in an amount suicient to provide said water with a maximum pH of 1, and a suicient amount of a material capable of functioning as an emulsifying agent in the presence of Water having said pH value, agitating the mixture at suitable time and temperature until the mixture becomes a single clear phase as a solution, adding to the solution a watersoluble inorganic metal salt capable in suicient amount of increasing the ionic strength of said water, continuing the agitation until dispersed oily droplets separate from the solution, providing further agitation of the mixture until said droplets harden to form solid, hard, substan- 3,672,833 Patented June 27, 1972 ice tially spherical beads, and separating said solid beads from said mixture, said addition of the ionizable salt being made in an amount suicient to provide hardening of said droplets between the time of formation of the clear solution and the completion of said further agitation whereby the presence of said salt during said further agitation assists in the hardening of the droplets to form said solid beads.

However, it appears from the above that the structure of the resulting beads is not that of an aerogel.

It has been further proposed to produce a dry aerogel by a method which comprises the ,steps of first precipitating a colloidal substance in a liquid as a gel, substituting for the iirst liquid (for example, water) a second liquid having a lower critical temperature, confining the resulting product in -a pressure vessel, applying heat thereto until the liquid in the gel has reached a temperature at which the surface tension of the liquid is materially reduced whereby to reduce the ultimate shrinkage of the gel when the liquid is allowed to evaporate, maintaining such temperature, and then releasing the vapor from the pressure vessel at a rate insufficient to injure the gel.

As it will appear from the following, the method according to the invention olers a number of advantages over the known processes, particularly with regard to the fastness and simplicity of carrying it out. It is not necessary to handle the gel while it is still immersed in the solvent. Moreover, due to the fact that the by-products of the reaction are volatile in the conditions of treatment applied, any washing of said gel is superuous.

The method according to the invention of preparing silica aerogels by hydrolyzing a lower alkyl orthosilicate comprises dissolving the same in a lower, water-miscible alkanol, subsequently adding to the resulting solution water, adding in a separate vessel a supplementary amount of said lower alkanol, gradually heating the contents of both vessels in an autoclave until the temperature reached is above the critical point, then gradually releasing the pressure and flushing out the water and alkanol vapours with a stream of a dry, inert gas and again lsealing the autoclave and cooling it until the ambient temperature is reached, whereupon the resulting aerogel is collected.

Under the wording of a lower alkyl orthosilcate is meant an orthosilicate the alkyl group of which contains one or two carbon atoms; i.e. either methyl orthosilicate or ethyl orthosilicate. Both compounds are commercially available.

Under the wording of a lower, water-miscible alkanol is meant an aliphatic alcohol containing one to four carbon atoms bound as an alkyl chain which is either straight or branched. In other words, said alkanol can be selected from the following ones: methanol, ethanol, n-propanol, isopropanol, n-bntanol, isobutanol, sec.butanol, and tert.- butanol.

Advantageously, the amount of alkyl orthosilicate used is substantially comprised between 5 and 30% by volume of the quantity of alkanol. Preferably, the quantity is of the order of 10%, the proportion which has given the best results.

Hydrolysis of the above delined mixture can be effected either in an acidic, .neutral or basic medium. In the iirst case and by virtue of its volatile nature, it has been found preferable to use acetic acid as acidifying compound. In the latter case and for the same reason, ammonia is used as alkalising compound.

The amount of water used to hydrolyse the lower alkyl orthosilicate is advantageously comprised between one and ve times the stoichiometric quantity when the medium is acid (by adding acetic acid) or neutral; if the medium is basic (by adding NH3), gelitication occurs at the ordinary temperature when the quantity of water is more than twice the stoichiometric quantity. 'This premature gelification is not a disadvantage.

The stoichiometric quantity of water is calculated after the equation solution of alkyl orthosilicate have been introduced. The

temperature is then raised in as linear a manner as possible until it is above the critical temperature of the solvent after which the' solvent vapour is evacuated. When pressure has returned to normal, the autoclave is flushed out with an inert, dry gas in order to remove all the vapours that can condense. The temperature is then brought back to normal, after the autoclave valves have been closed to prevent any atmospheric water vapour from condensing.

A more detailed description will now be given of various embodiments of the invention with reference to the accompanying drawing, which is a diagram of apparatus for working the method.

The apparatus comprises an autoclave 1 which can be closed by a lid 2 and is connected by a pipe 3 via drying columns 6 to an inert gas reservoir 5, and by a pipe 4 to a vessel 7 for holding the solvent, after it has condensed in a condenser 8. Valves 9 and 10 are disposed on pipes 3 and 4 respectively. A thermoprobe 11 and a pressure gauge 12 show the variations of temperature and pressure inside `500 ml. methyl orthosilicate were dissolved in 4,500V

,m1, methanol. Theresulting solution contained approxmately 10% of methyl orthosilicate-The solution was cooled to 0 C., after which a quantity of water was added equal to from two to :live times the stoichiometric quantity Y required for reaction 1. i Y

Si(OCH3)4-i-2H2O SiO2+4CH3OH 1*) This solution was poured into a vessel which was placed in autoclave 1, which had an intern-al volume of 3001. An-

other vessel containing 95 l. methanol was placed inside the autoclave along with the vessel containing the solution. 'I'he quantity of methanol was suticient for the critical pressure to -be exceeded when the temperature rose y above the critical point.

The autoclave was sealed and the temperature was raised from 20 C. to 260 C. (18 C. above the critical temperature) in 2 hours 30 minutes, the temperature being increased at the rate of 1.6 C./minute. At the end of this period, the methanol vapour was evacuated byl f ing a volume of about 5 l. The whole process lasted about 4 hours.

Aerogels obtained in the manner cited are remarkable for their porosity. The porosity can be studied e.g. by

adsorptionof gases at low temperature and by mercury porosmetry. A H

It has been shown that,:1in general, the adsorption of argon at -195 C. can be used to determine the volume of the mieropores (pore radiusless than A.). The

"volume of pores measuredA by theadsorption of nitrogen at the same temperature is equal to the Ivolume of the micropores plus thevolume ofthe middle-sized pores (pore radius 420 A.). The volume of pores' determined by mercury penetration is equal to the volume of the macropores'(pore radius 290'A.).`I s i The general recationfor the precipitation of silica is given by cited Equation 1. Actually, however, the substance precipitated is arelatively polycondensed polysilicic acid. AAs a result, the resulting aerogel does not have the formula Si02 but rather theformula SiOz, nHzO, with n 2. In the case where n=2, the reactionis limited to the formation of monomeric "silicicl acid Si(OH)4, according toEquationZ: p f g- Si(OCH3)+4H2O Si(OH)4i-4CH3OH i y (2) Since the polycondens'ation of the afore-mentioned acid, which gives silica lgel S102 is accompanied by the elimination of water, aerogels prepared under stoichiometric conditions corresponding to reaction 2 lose a little water when treated in the autoclave. I' 7v l It has been shown that in an acid'medium- (0.175 N acetic acid) precipitation is *very incomplete when the exact stoichiometric quantity off'water is used (Equation 1). 'Ihe aerogel obtained in these conditions by hydrolysis of a 10% solution'of neth'yl orthosilicate in methanol (gel E1) has a very low porosity compared with aerogels obtained with anexcess of water. When the quantity of water used is twice the stoichiometric quantity for reaction l, thevresulting aerogel (No.2.) occupiesthe complete volume of the alcoholicy methylv vorthosilicate solution initially placed in the autoclave, and its texture is considerably improved'. Gels have been prepared having a total porosityhof the order of 18 tzm/g.l (gel No. 3). When the concentration of water isexcessive (gel No. 4),

there vis a marked decrease in porosity. v Y l The results in thefollowing Table I show that all the textural v properties of -.aerogels obtained by the cited method pass through a maximum when the quantity of `water used is between two and livetimes the stoichio- `metric quantity lfor reaction 1,..i.e. when the ratio H2O/S1(OCH3)4` 1s between 4 and 10. ,1. f

TABLE I A(Precipitation in an acid medium) Aero ele l i HIOA Y S I V Y g xampe`- A-V 4V No. suoona), m.2/g. cmi/gf emi/gi, cm/I 2 aso o. 15 o. 1s 2. 7 4 865 o. 72 1. s2 11. 6 1o 68s 0.62 1.72 17.3 20 405 0.40 1.52 7.2

In lthe table:

S=speciic surface measured with nitrogen by the B.E.T. method (cf. S. Brunauer, P. H. Emmett and p freut-1,1. Chem. soc. (19138), 6o, 15.309)` VDA-:volume of poresmeasuredwithzargon. VpN=volume of pores measured' with nitrogen, and Vp=volume of pores determined by mercury penetration.

`ment occurs in a neutralmedium. The' porosity accessible -to adsorbed gases (VPA and VDN)fvarie'sfin` depende/nce on the quantity'of waterintroduced, in fthe same Vmanner V:is in an acid medium'. on :heotherhaa,fhevo1ume f the large pores (Vp) is not affected by a large excess of water (Table Il).

In a basic medium (0.5 N ammonia), the precipitation is considerably less than in a neutral or acid medium, even when the water is restricted to the stoichiometric quantity as shown by the weights of gel obtained.

The initial silica sol obtained under the last-mentioned conditions is much less stable, and as soon as the concentration of water is more than twice the stoichiometric quantity for reaction 1, some of the silica precipitates as soon as methyl orthosilicate is introduced into the hydrated methanol. For this reason, the effect of the water concentration in an alkaline medium cannot be studied if the ratio H2O/Si(OCH3)4 is greater than 4. It is still possible to prepare aerogels, however, if the substance introduced into the autoclave is not a homogeneous liquid solution but a heterogeneous solution which has been partly geliiied or precipitated.

The results in Table Ill show that aerogels obtained in an alkaline medium have considerable lower macroporosity (VP) than aerogels precipitated in a neutral or acid medium although the microporosity (VDN and VPA) is comparable. The aerogels have a very peculiar appearance. Aerogels prepared by the cited method in a neutral or acid medium are white and opaque, but aerogels obtained in an alkaline medium are colourless and completely transparent, even up to a thickness of approximately 1 centimetre.

A study was also made of the etfect of the concentration of methyl orthosilicate in a slightly acid medium (0.175 N acetic acid) in the presence of a quantity of water equal to the stochiometric quantity for reaction 2.

In the table:

dg=apparent density of solid.

The results in Table lV show that in the case of an aerogel prepared from a solution of 5% by volume of methyl orthosilicate in methanol microporosity and macroporosity are considerably lower than for the other solids. This is due to the fact that, if the concentration of methyl orthosilicate is less than the solid withdrawn from the autoclave occupies a considerably smaller volume than the initial solution. However, as soon as the concentration reaches 10%, the dry gel occupies very nearly the volume of the initial solution, with the result that its apparent density is substantially proportional to the concentration of methyl orthosilicate, as sho-wn by the experimental results given in the last column of Table 1V.

As can be expected, the pore volumes, measured by gas adsorption or mercury porosimetry, decrease when the concentration of methyl orthosilicate increases from 10 to 30%. On the other hand, the specific surface is not much aiected by the last-mentioned factor (Table 1V).

A study was also made of the thermal stability of aerogels prepared according to the invention. To this end, an aerogel precipitated by hydrolysis of a solution of 10% methyl orthosilicate in a 0.175 -N acetic acid medium in the presence of a stoichiometn'c quantity of water for reaction 2 was thermally treated for 8 hours in air at temperatures between 300 and I900 C.

It was found that heating to 500 C. considerably irnproves the macroporosity of aerogels when the measure is effected in mercury (16.5 cm/ g. instead of 11.1). The volume of pores accessible to mercury was not noticeably lower than that of the initial solid until the temperature of treatment exceeded 700 C.

On the other hand, the porosity accessible to adsorbed gases (VPN and VPA) decreased progressively when the temperature increased. The results are shown in the following Table V.

TABLE V Effect of thermal treatment for 8 hours in air on the texture of a silica aerogel prepared from methyl orthosilicate The cited data show that the macropores of silica aerogels prepared from methyl orthosilicate have good thermal stability up to 700 C.

The pore volume of aerogels treated to 700 C., measured by mercury penetration, is not appreciably lower than for the aerogel used as starting material, in spite of the marked decrease in porosity accessible to adsorbed gases (VPN, VPA and S). The apparent density and appearance of the aerogel are also practically unchanged, provided the heating temperature does not exceed 700 C. The afore-mentioned solids are particularly suitable as heat insulators over a very wide temperature range 200" C. to +700 C.).

Aerogels prepared in an autoclave in the presence of a lower alkanol are relatively hydrophobic, as silicic acid is then re-esteried to some extent by said alkanol. They absorb only a moderate amount of water vapour and when the re-esteri-lication rate is considerable, for instance after prolonged heating under pressure, they are not wetted by Water.

The following examples relate to a second embodiment of this invention wherein the alkyl orthosilicate is ethyl orthosilicate.

Example 14 (hydrolysis in neutral medium) 25 gramsethyl orthosilicate were dissolved in 225 grams ethanol. The resulting solution contained approximately 10 percent by weight of ethyl orthosilicate. The solution was cooled to 0 C., after which 18 grams water were added, a quantity which amounts to -8 moles water per mole ethyl orthosilicate. The solution was introduced in an autoclave which was sealed immediately and progressively heated within three hours, from 20 C. to 290 C., the temperature being increased at the rate of 1.5 C./minute.

As soon as the latter temperature was reached,the ethanol vapour was evacuated. When temperature had returned to ambient temperature under purging the autoclave by blowing dry nitrogen therethorugh, the` dry aerc'gel was withdrawn. Its specific surface was SN=815 m./g. (as measured by nitrogen adsorption) and its volume of pores was VP=15 ml./g. f

Example (hydrolysis in acidic medium) The following examples Irelate to the use of lower alkanols other than those of which the orthosilicates to be used are derived, that is, other than methanol and ethanol.

Example 16 grams methyl orthosilicate were dissolved in 225 grams n-propanol. The resulting alcoholic solution contained 10 percent by weight of silicium alkoxide. To this solution were added 24 grams water, i.e. 8 moles per mole of methyl orthosilicate.

The treatment was effected in an autoclave in the same manner as described in the foregoing examples.

The silica aerogel thus obtained had a specific surface S=860 m/g. and a volume of pores Vp=l7.5 crn/g.

Example 17 The operation was carried out in the same manner as described in Example 16 with the exception that npropanol was replaced with a same amount of isopropanol. f if 1 The silica aerogel thus obtained had a specific surface S=680 m.2/g. and a volume of pores Vp=16.1 cm/ g.

. Example 18 l The reaction was carried out inthe manner described in Example 16 with the exception that n-propanol was replaced with a'same amount of n-butanoli'A silica aerogel wasobtained which had the following texture:`

The reaction wascarried out in the manner described u in Example 16 with the exception that n-propanol was replaced with a same amount of sec.butanol. A silica aerogel was obtained which had the following texture: 'e o SU=8H60 m.'/g.; \{'1=117.2 cm/g.V

' Example -20 The reaction was carried out in the manner described in Example. 16 with the exception that n-propanol was replaced with a same amount of tert.butanol.

, A silica aerogel Iwas obtained which had the' following textura' -f Y Y f S=635 m.2/g.; /Vp=15v.1 cm.3/g.

Thelabove-described lExamples 14 to 20 ha`e been carried out in the manner of Example l by placing inside the autoclave a separate vessel containing an amount of the aliphatic alcohol employed, such that the critical pressure is exceeded when the temperature rises abovethe critical point.

y The silica aerogels prepared in the above-described manner all have the property of gelifying organic liquids or aqueous solutions when added in very small proportions (approximately 5 percent by weight of said liquids or solutions. They can also beused to increase the viscosity of such liquids.

Whatever embodiment, therefore, is used, a method is obtained for preparing silica aerogels having properties which have been suiciently described hereinbefore and which have numerous advantages over known aerogels, inter alia the elimination of the operations ofwashing the gel and of substituting a solvent for the water impregnating the gel.

The invention, of course, is not limited to the cited embodiments but covers all variants.

The invention is hereby claimed as follows.

We claim:

1. A method of preparing silica aerogels by hydrolyzing a lower alkyl orthosilicate which comprises dissolving the same in an aliphatic alcohol having one to four carbon atoms, subsequently adding to the resulting solution water in an amount at least equal to the amount required to hydrolyze the lower alkyl orthosilicate, adding in a separate vessel a supplementary amount of said alcohol, gradually heating the contents of both vessels in an autoclave until the temperature reached is above the critical point, then gradually releasing the pressure and flushing out the water and alcohol vapors with a stream of a dry, inert gas and again sealing the autoclave and cooling it until the ambient temperature is reached, whereupon the resulting aerogel is collected. l

2. A method according to claim 1, wherein said lower alkyl orthosilicate is selected from the group consisting of methyl orthosilicate and ethyl orthosilicate.

3. A method according to claim 2, wherein said lower alkyl orthosilicate is methyl orthosilicate.

4. A method according to claim 1, wherein said alcohol is methanol.

5. A method according to claim 1, wherein said alcohol 6. A method according to claim 1, wherein said alcohol is n-propanol.

7. A method according to claim 1, wherein said is isopropanol.

8. A method according to claim 1, wherein said alcohol is n-butanol.

9. A method according to claim 1, wherein said alcohol is isobutanol.

10. A method according to claim 1, wherein said alcohol is see-butanol. v f

11. A method according to claim 1, wherein said hol is tert.butanol. e f 12. A method according to claim 1, wherein said dry inert gas is nitrogen. f

13. A method according to claim 3, wherein said dry inert gas is nitrogen.

14. A method according to claim 1, wherein hydrolysis is eected in neutral medium.

15. A method according to claim 1, wherein hydrolysis is eiected in acidic medium.

16. A method according to claim 1, wherein hydrolysis is effected in the presence of acetic acid.

17. A method according to claim 1, wherein hydrolysis is eiected in alkaline medium.

18. A method according to claim 1, wherein is effected in the presence of ammonia. 19. A method of preparing silica aerogels by hydrolyzing methyl orthosilicate which comprises dissolving the same in methanol, subsequently adding to the resulting solution water in an amount at least equal to the stoichiometrical amount, adding in a separate vessel a supplementary amount of methanol, gradually heating the con tents of both vessels in an autoclave until the temperature alcohol alcohydrolysis vreached is above the critical point, then gradually releasing the pressure and llushing out the water and methanol 9 10 vapors with a stream of a dry, inert gas and again seal- 3,165,379 1/ 1965 Schwartz 23-182 R ing the autoclave and cooling it until the ambient tem- 2,269,059 1/ 1942 MCLaChlaU, JL 23-132 R X perature is reached, and then collecting the resulting 3,321,276 5/1957 BUfZyDSk et al. 23-182 R aerogeL 3,354,095 BuI'ZyIlSki et al. 252--316 20. A method according to claim 19 wherein said dry 5 3,146,252 8/1964 Emblem et al 26o-4483 inert gas is nitrogen. Klstler R 2,285,449 6/ 1942 Marshall 23--182 R UX References Cited 2,757,073 7/1956 Drexel 23-182 R UNITED STATES PATENTS 10 EDWARD STERN, Primary Examiner 3,236,594 2/1966 Ray 23-182 R U S CL X.R

3,131,072 4/ 1964 Taulli 10G-306 252-315, 317

UNITED STATES PATENTv OFFTCE CERTIFICATE 0F CGR ECTTN 833 June Z7, 1972 Patent No. 3 672 Dated nventor(s) Stanislas Jean Teichner et al It is certified that error appears in the above-identified patent and that' said Letters Patent are hereby corrected as shown below:

In the heading of the patent, insert Claims priority, application France, Nov. 30, 1967 PV 130,147

Signed and sealed this 3rd day of April 1973,

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner oi:v Patents FORM Do-1050 (1o-69) USOMM.DC @O3-54:69

U`S. GOVERNMENT PRINTING OFFICE: i969 0*-356-334,

UNTTED STATES PATENT oTTTcE CERTWC/XTE 01' CGRRECTEON Patent No. 3,672 ,833 d Dated June 27 1972 Inventor(s) Stanislaus Jean Teichner et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading of the patent, insert Claims priority, application France, Nov. 30, 1967 PV 130,417

This certificate supersedes Certificate of Correction issued April 3, 1973.

Signed and sealed this 20th day oi November 1973,

(SEAL) Attest:

EDWARD M.FLETCHER,JR. Y RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents FORM P04050 (10'69) uscoMM-Dc sone-Pes u s. GOVERNMENT PRN'NNG OFFICE i 1969 0-36"334, 

